Occult primary tumors are defined as histologically proven
metastatic malignant tumors whose primary site cannot be identified during
pretreatment evaluation. Even after postmortem examination, the primary
tumor is not identified in 20%-50% patients. Occult primary tumors, or
cancers of unknown primary site, account for 5% to 10% of all diagnosed
cancers. These tumors are manifested by a wide variety of clinical
presentations. For most patients with occult primary tumors, the disease
is incurable. Patients with cancer of unknown primary site demonstrate
common characteristics and present with general complaints such as
anorexia, weight loss, etc.

Multiple sites of involvement are shown in more than 50%
of

patients with occult
primary tumors. The common sites of involvement are the liver, lungs,
bones, and lymph nodes. Clinical absence of primary tumors, early
dissemination, aggressiveness, and unpredictability of metastatic pattern
are characteristic of these tumors. Life expectancy is very short with a
median survival of about 6-9 months. Patients with occult primary tumors
often have chromosomal abnormalities in the short arm of chromosome 1.
Abnormalities in the p53 gene have been identified in more than 70% of
patients with unknown primary tumors.

There are two widely held misconceptions regarding occult
primaries. First, both patients and physicians think that if a primary
cancer is found, an effective chemotherapy can be identified, which will
significantly improve survival. In fact, for most solid tumors that have
metastasized, chemotherapy is only palliative and does not significantly
improve long-term survival. Secondly, the statement

“common things
are common,” suggests that knowing the patterns of metastases will
identify the primary sites. While it is true that certain patterns of
metastases suggest possible primaries, occult primaries can metastasize to
any site. Therefore, one should not rely on patterns of metastases to
determine the primary site.

Occult primary tumors occur equally in men and women,
usually in the sixth decade of life. A primary tumor site is found in
fewer than 30% of patients who present initially with an occult primary
tumor. At presentation, half of patients with an occult primary tumor have
multiple sites of involvement such as lymph nodes, lung, bone, liver,
pleura, and the brain. Poor prognostic factors associated with an occult
primary tumor include male gender, a pathologic diagnosis of
adenocarcinoma, the presence of hepatic involvement, and an increasing
number of involved organ sites.

Most of these patients are treated with chemotherapy... radiation is
useful for areas of symptomatic metastases (see
here) and is commonly used where the source is thought to be the
breast (see here)
and on lymph node metastases to the neck (cervical) nodes (see
here) or inguinal (groin) nodes (see
here.) see review of
squamous
cancers here.